Abstract

Hardened underground structures are purposely constructed beneath the ground
surface
for additional protection. The protection and sustainability of such a structure
is provided by the soil overburden, supported by the structure's roof slab, also called a
primary slab. The soil overburden usually contains one or more
layers of protective
reinforced concrete slabs, defined as secondary slabs. Hardened sub-surface structures
can experience a degree of damage from
sub-surface explosions, caused by the
detonation of airborne weapons at variable depths within the soil overburden
layer.
The
research objective was
to evaluate
the degree to which material and geometric
parameters
influenced the magnitude of groundshock
loading and hence the severity
of damage to sub-surface positioned reinforced concrete slabs.
The work was of an experimental nature, performed within a purposely built test cell.
The experimental set-up
involved positioning cylindrical charges within the soil
overburden at variable standoffs
from an
initially positioned steel plate,
then from a
primary slab, which was subjected
to a series of cumulative
loadings. The structural
response of both the steel plate and a reinforced concrete slab were evaluated using an
energy balance procedure.
A parametric study was performed, determining the influence each geometric
parameter had to the magnitude of groundshock
loading. The variability of data
associated with the material parameters was analysed and compared
to the published
literature.
Numerical simulations were performed at
the end of the experimental stage
(using the
non-linear
finite element programme AUTODYN21)) to investigate phenomena
that
could not be investigated experimentally.
A visual damage assessment
in the form of a crack pattern analysis and chronological
order of occurring mechanisms upon reinforced concrete slabs was performed. The
severity of damage to the primary slabs was
then associated with
the cumulative
load
impulse history.
The
research yielded
the following conclusions:
I. An idealised half sine-wave
load distribution approximated
the pressure-time
history profiles recorded by pressure gauges.
2. The steel plate and one concrete slab
(for which conclusive data was obtained)
responded
impulsively to groundshock
loading.
3. The magnitude of groundshock
loading was most sensitive
to a change
in the
charge standoff.
4. The
reduction
in the soil overburden above the top face of a secondary slab did
not
influence the groundshock
loading induced into a primary slab.
5. The effect of the propagating groundshock wave overrode any
initial form of
soil compaction between the charge and
target. This was due to the
significantly high stresses
induced into
the soil, which were well above the
initial insitu
stresses.
6. Internal weakening of a primary slab, which was unidentifiable
from the
external damage, caused significant
loss in
structural strength and stiffness.